IS forces began a major offensive in Hasakeh in February, seizing several villages and kidnapping several hundred members of the Assyrian Christian minority in the area.

But it has also suffered setbacks, including the February 27 loss of jihadist bastion Tal Hamis.

The extremist group is now battling to take Tal Tamr and the town of Ras al-Ain, which sits on the border with Turkey.

Tal Tamr lies on a road leading to Ras al-Ain, as well as a highway running across the Iraqi border to the IS stronghold of Mosul.

The coalition fighting IS in Syria and Iraq has provided air support for Kurdish fighters battling the group in northern Syria, and was key to their holding on to the strategic border town of Kobane, which they almost lost.

The coalition said it had not carried out any strikes in Syria between Wednesday and Thursday.

YPG spokesman Redur Khalil told AFP that IS was bringing in additional forces to the Ras al-Ain area from its strongholds in Raqa city and Tal Abyad.

He said Kurdish forces were expecting an "imminent attack" in the area around Ras al-Ain, which is north of Tal Tamr.

"After losing Tal Hamis... Daesh is trying to restore its prestige in the area as much as possible," Khalil said, using an Arabic acronym for IS.

The Syrian Observatory for Human Rights, a Britain-based monitor, said at least 35 YPG fighters had been killed in three days of clashes in the area, but had no toll for IS forces.

IS controls large swathes of territory in Syria and Iraq, and has declared the area under its control an Islamic "caliphate".

More than 210,000 people have been killed in Syria's conflict, which began in March 2011 with anti-government protests and later escalated into a civil war that has attracted thousands of foreign jihadists.